[concurrency] add executable test of MainActor

Author: kavon

test/Concurrency/Runtime/mainactor.swift

@@ -0,0 +1,44 @@
+// RUN: %target-run-simple-swift(-Xfrontend -enable-experimental-concurrency %import-libdispatch) | %FileCheck %s
+
+// REQUIRES: executable_test
+// REQUIRES: concurrency
+// REQUIRES: libdispatch
+
+// XFAIL: *
+
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Glibc)
+import Glibc
+#endif
+
+import Foundation
+
+// CHECK: starting
+// CHECK-NOT: ERROR
+// CHECK: hello from main actor!
+// CHECK-NOT: ERROR
+// CHECK: ending
+
+@MainActor func helloMainActor() {
+  if Thread.isMainThread {
+    print("hello from main actor!")
+  } else {
+    print("ERROR: not on correct thread!")
+  }
+}
+
+func someFunc() async {
+  await helloMainActor()
+}
+
+runAsyncAndBlock {
+  print("starting")
+  let handle = Task.runDetached(operation: someFunc)
+  do {
+    try await handle.get()
+  } catch {
+    print("ERROR: exception was thrown while waiting for task to complete")
+  }
+  print("ending")
+}